Characterization of Thermal Resistance Coefficient of High-power LEDs

Heat at the junction of light-emitting diodes (LED) affects the overall performance of the LED in terms of light output, spectrum, and life. Usually it is difficult to measure junction temperature of a LED directly. There are several techniques for estimating LED junction temperature. One-dimensional heat transfer analysis is one of the most popular methods for estimating the junction temperature. However, this method requires accurate knowledge of the thermal resistance coefficient from the junction to the board or pin. An experimental study was conducted to investigate what factors affected the thermal resistance coefficient from the junction to the board of high-power LED. Results showed that the thermal resistance coefficient changed as a function of ambient temperature, power dissipation at the junction, the amount of heat sink attached to the LED, and the orientation of the LED with the heat sink. This creates a challenge for using onedimensional heat transfer analysis to estimate junction temperature of LEDs once incorporated into a lighting system. However, it was observed that junction temperature and board temperature maintains a linear relationship if the power dissipation at the junction is held constant.